FscryptInstallKeyring: don't re-create keyring if it's already created

During userspace reboot FscryptInstallKeyring will be called again, this
CL will make it second call a no-op, which IMHO is better than having a
special logic in init to conditionally call FscryptInstallKeyring
depending on whenever it's normal boot, or userspace reboot.

Test: adb reboot userspace
Test: checked in kernel logs that new keyring is not created
Bug: 135984674
Change-Id: I4ad5aee6887b7318fb1cd02bf1c7be8da6ece599